Phase evolutions and growth kinetics in the Co–Sn system
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Co/Sn diffusion couples are annealed at 175–220 °C. The reaction phase CoSn3 is found to have a narrow homogeneity range of 2 at% with composition 74–76 at% Sn. The growth of this phase is found to be reaction–controlled initially followed by diffusion–controlled later. The marker experiment indicates that Sn is the faster diffusing element in this phase. The CoSn2 phase is found to grow in the Co/CoSn3 incremental couples at 200–220 °C.
KeywordsCo–Sn phase diagram Intermetallic compounds Incremental diffusion couple Electron probe micro analyser (EPMA)
2 Experimental procedure
Pure Co (99.95 wt%) and Sn (99.99 wt%) were used as starting materials to make diffusion couples. They were prepared metallographically to obtain a flat and smooth surface. Then, they were cleaned in acetone and dried. The couple halves were joined by clamping in a special fixture with minimum required external pressure. Diffusion couples were annealed in the temperature range of 175–220 (± 2) °C for 100 h in a calibrated oven in high vacuum (~ 10−4 Pa). Prior to annealing at 200 °C, more or less evenly distributed Y2O3 particles (average size of 4.4 µm) were introduced at the contact interface to act as inert ‘Kirkendall’ markers. The bonded specimens were removed from the oven and cross sectioned by a slow speed diamond saw. After the standard metallographic preparation, the interdiffusion zone was examined in a scanning electron microscopy (SEM) and the composition profiles were measured in electron probe micro−analyzer (EPMA) equipped with a field emission gun (FEG). The location of the marker plane was identified in electron dispersive spectrometer (EDS) with the help of X–ray peak originating from Y and O.
3 Results and discussion
Co/Sn diffusion couples are studied in the higher temperature range 220–175 °C because of the joining issues at 150 °C and below. The maximum temperature for the annealing was selected based on the melting point of Sn, so as to meet our objective of studying solid/solid interactions. Before discussing results on the evolution of phases, it is important to refer the Co–Sn phase diagram shown in Fig. 1 [3, 4]. It is be noted here that in this latest binary phase diagram, the newly found CoSn3 phase is added. Total 4 phases viz. Co3Sn2, CoSn, CoSn2 and CoSn3 are present in the phase diagram.
3.1 Phase evolutions
Further, the line profiles measured (in EPMA) show the similar trend at all the temperatures. Solid circles in the graph (Fig. 2b) show the compositions of the phases reported in the phase diagram, whereas the open squares represent the actual compositions measured in EPMA. It indicates that the dissolution of other elements in the end–members is minor or negligible. It also indicates the off–stoichiometry in the composition of CoSn3, which has narrow homogeneity range composition, measured as 74–76 at% Sn. Diffusion couple technique (which is employed in this study) is one of the reliable techniques to validate the phase diagram , in particular, when the composition does not change sharply near the phase boundaries (like in the present system) in the interaction zone. By careful point analysis in an EPMA equipped with FEG, we have measured the phase boundary compositions (PBC) within 2–3 µm from the interface since the interaction volume is ~ 1 µm. Moreover, the PCB measured, in this manner, when compared with the line profiles (e.g., Fig. 2b) are found to be same. In past, diffusion couple technique is used to validate PBC in Co–Ta , Co–Mo  and Au–Sn  systems, in a similar fashion as done here.
3.2 Growth kinetics of the CoSn3 phase
Hence, to gain further insights on the growth mechanism, inert Y2O3 particles were used as markers at the contact interface of Co/Sn prior to annealing at 200 °C. After the annealing treatment, the location of the marker plane is found at CoSn3/Sn interface, marked as “K” in Fig. 2c. This indicates that the growth of CoSn3 phase would be mainly by the diffusion of Sn atoms (faster diffusing species), and the diffusion of Co atoms through the CoSn3 phase layer could be considered as negligible. In this study, based on the time dependent experiments, we have found transition from reaction–controlled to diffusion–controlled growth. It means that during initial stages of the growth of CoSn3, the phase layer thickness is thin enough such that Sn atoms can diffuse easily across the layer. Thus, the growth rate is limited mainly by the chemical reactions which occurs at the interface, and the layer thickness increases linearly with the holding time. This means that the diffusion rate is relatively faster at the beginning of the phase layer growth, and the formation of the reaction phase (i.e., CoSn3) is the rate–controlling process. The time for diffusion of species across the phase layer would increase with the thickness. Therefore, later (i.e., after holding time of say 100 h) when the layer thickness increases, it would reduce the supply of Sn atoms at Co/CoSn3 interface (growth front) and since diffusion rate has decreased, now the growth of phase is parabolic with the duration of holding time. This means that the diffusion rates of components through the reaction phase (i.e., CoSn3) is the rate–controlling process, and the rate of formation of the reaction phase is relatively faster. Since the main barrier for the reaction to take place now changes to the atomic diffusion, there is transition in the growth mechanism of CoSn3 phase from reaction–controlled to diffusion–controlled.
Only CoSn3 phase, which has been recently added in the latest Co–Sn binary phase diagram available in the literature, is found to grow in the Co/Sn diffusion couples annealed at 175–220 °C.
Based on the binary phase diagram, 3 more phases are expected to grow in the interaction zone. In the Co/CoSn3 incremental diffusion couples, an additional phase CoSn2 is found to grow at 200–220 °C.
CoSn3 is found to have a narrow homogeneity range composition, i.e., 74–76 at% Sn, based on line profiles measurements and spot analysis in an EPMA equipped with FEG.
The growth of CoSn3 phase is found to be reaction–controlled upto 100 h with growth exponent n = 1.12 followed by diffusion–controlled till 625 h with n = 0.47 in this study. Sn is found to be faster diffusing element based on inert marker experiments.
Author, V.A.B., would like to thank Professor Aloke Paul (Department of Materials Engineering, Indian Institute of Science, Bangalore) for his support. The research is funded by DST (Department of Science and Technology, India) under INSPIRE (Innovation in Science Pursuit for Inspired Research) Programme.
Compliance with ethical standards
Conflict of interest
The author declare that he has no conflict of interest.
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